Hybrid vehicle and associated control method

ABSTRACT

A hybrid vehicle and method of control include an internal combustion engine and at least one traction motor operated in an automatic mode such that the combined wheel torque is a function of accelerator pedal position and vehicle speed. In a select shift mode, wheel torque is also a function of a virtual gear number. The virtual gear number varies in response to driver activation of shift selectors or automatically in response to changes in vehicle speed. The vehicle transitions into select shift mode in response to driver activation of a downshift selector. When the transition occurs, an initial virtual gear number is selected to ensure that wheel torque increases.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.13/539,945 filed Jul. 2, 2012, now U.S. Pat. No. 8,834,317, thedisclosure of which is incorporated in its entirety by reference herein.

TECHNICAL FIELD

This disclosure relates generally to controlling the engine speed andcombined output torque of a hybrid vehicle in response to driver inputs.

BACKGROUND

In a vehicle having a discrete ratio transmission, the speed of thetransmission input shaft is constrained to be proportional to thevehicle speed with a finite set of ratios, except during the briefinterval while the transmission is shifting from one ratio to anotherratio. When the torque converter is locked, the engine speed is alsoconstrained to be proportional to vehicle speed. In a hybrid electricvehicle having a power-split architecture, on the other hand, thetransmission does not mechanically impose a strict relationship betweenthe engine speed and the vehicle speed.

Even in vehicles with automatic transmissions, in which selection of thegear ratio or engine speed is ordinarily determined by a controller,some drivers prefer to occasionally over-ride the controller to provideoperation similar to a manual transmission. Some vehicles are equippedwith shift paddles or other driver interface features which permit thedriver to signal a desire for a higher or a lower gear ratio relative tothe gear ratio automatically selected by the vehicle controller, with anassociated change in engine speed and vehicle torque. In a discreteratio transmission, the controller responds to such a command byshifting to a different one of the discrete gear ratios, which adjustsengine speed accordingly and provides associated torque multiplicationat the vehicle wheels. However, in a vehicle with a continuouslyvariable transmission or similar gearbox, such as a power-split hybrid,the response is more complicated because the transmission does notinherently provide discrete gear ratios with associated different torquemultiplication.

SUMMARY

In various embodiments, a hybrid vehicle control strategy implementsfour different operating modes. The vehicle controller determines whichoperating mode is utilized at any given time in response to operation ofvarious driver interface elements including a shift lever, a downshiftselector, and an upshift selector, for example. In two of the operatingmodes, the controller permits the driver to select a virtual gear thatimpacts the engine speed and the combined output torque of the engineand one or more traction motors. The controller can utilize differentlogic for shutting the engine off and driving solely with electric powerdepending on which operating mode is active.

In one embodiment, a method of controlling a hybrid vehicle includescontrolling an engine and a traction motor in an automatic mode and thenincreasing the wheel torque to transition to a select shift mode. In theautomatic mode, the wheel torque is based on pedal position and vehiclespeed. In the select shift mode, the wheel torque is based on a drivermodifiable virtual gear number in addition to pedal position and vehiclespeed. In the select shift mode, the wheel torque decreases as thevirtual gear number increases. The method may include increasing thevirtual gear number in response to operation of an upshift selector anddecreasing the virtual gear number in response to operation of adownshift selector. The transition to select shift mode may be initiatedby operation of the downshift selector. When transitioning to the selectshift mode, the method may include selecting the highest initial virtualgear number that will result in a torque increase.

In another embodiment, a controller for a hybrid vehicle includes inputcommunication channels, output communication channels, and controllogic. The input communication channels receive input signals indicatingvehicle speed, position of a driver operated accelerator pedal, driveroperation of a downshift selector, and driver operation of an upshiftselector. The output communication channels send control signals to anengine and at least one traction motor. The control logic is configuredto control the engine and traction motor(s) in an automatic mode andthen increasing the wheel torque to transition to a select shift mode.In the automatic mode, the wheel torque is based on pedal position andvehicle speed. In the select shift mode, the wheel torque is based on adriver modifiable virtual gear number in addition to pedal position andvehicle speed. In the select shift mode, the wheel torque decreases asthe virtual gear number increases. The controller may increase thevirtual gear number in response to operation of an upshift selector anddecrease the virtual gear number in response to operation of a downshiftselector. When transitioning to the select shift mode, the controllermay select the highest initial virtual gear number that will result in atorque increase.

In another embodiment, a hybrid vehicle includes a planetary gear setand a controller. The elements of the planetary gear set, which includea sun gear, a ring gear, and a planet carrier, are drivably connected toan engine, a set of driving wheels, and a first electric machine. Asecond electric machine is drivably connected to the wheels. Thecontroller is configured to control the engine and the electric machinesin an automatic mode and then increasing the wheel torque to transitionto a select shift mode. In the automatic mode, the wheel torque is basedon pedal position and vehicle speed. In the select shift mode, the wheeltorque is based on a driver modifiable virtual gear number in additionto pedal position and vehicle speed. When transitioning to the selectshift mode, the controller may select the highest initial virtual gearnumber that will result in a torque increase.

Various embodiments according to the present disclosure can provide oneor more advantages. For example, systems and methods for controlling ahybrid vehicle according to the present disclosure mimic or emulate amanual or select shift mode of an automatic step-ratio transmission in ahybrid vehicle having a continuously variable transmission or similargearbox. In addition, various strategies of the present disclosureprovide drivers of hybrid vehicles more interactive controls to manuallycommand powertrain speed and acceleration to provide enhanced luxuryfeatures and a sporty feel.

The above advantages and other advantages and features will be readilyapparent from the following detailed description of the preferredembodiments when taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a vehicle powertrain,controller, and user interface features of a representative embodimentof a hybrid vehicle according to the present disclosure;

FIG. 2 is a state transition chart illustrating operation of a system ormethod of an embodiment of the present disclosure;

FIG. 3 is a flow chart illustrating operation of a system or methodaccording to various embodiments when in a Normal operating mode;

FIG. 4 is a graph illustrating a relationship between vehicle speed,accelerator pedal position, and wheel torque command of a representativeembodiment according to the disclosure;

FIG. 5 is a graph illustrating a relationship between vehicle speed,target engine power, and engine speed command of a representativeembodiment according to the disclosure;

FIG. 6 is a flow chart illustrating operation of a system or methodaccording to various embodiments when in the Live-In-Drive (LID)operating mode;

FIG. 7 is a graph illustrating a relationship between actual acceleratorpedal position, virtual gear number or operating mode, and modifiedpedal position of a representative embodiment according to thedisclosure;

FIG. 8 is a graph illustrating a relationship between vehicle speed,virtual gear number, and engine power clipping limit of a representativeembodiment according to the disclosure;

FIG. 9 is flow chart illustrating operation of a system or methodaccording to embodiments of the disclosure when in the Sport operatingmode;

FIG. 10 is a flow chart illustrating operation of a strategy forshutting off and restarting the engine in certain operating modes ofvarious embodiments of the disclosure; and

FIG. 11 is a flow chart illustrating operation of a system or methodwhen in the SST operating mode according to various embodiments of thedisclosure.

DETAILED DESCRIPTION

As required, detailed embodiments of the present invention are disclosedherein; however, it is to be understood that the disclosed embodimentsare merely exemplary of the invention that may be embodied in variousand alternative forms. The figures are not necessarily to scale; somefeatures may be exaggerated or minimized to show details of particularcomponents. Therefore, specific structural and functional detailsdisclosed herein are not to be interpreted as limiting, but merely as arepresentative basis for teaching one skilled in the art to variouslyemploy the present invention.

A powertrain for a hybrid electric vehicle is illustrated schematicallyin FIG. 1. The powertrain includes an internal combustion engine 20driveably connected to a planet carrier 22, a generator 24 driveablyconnected to a sun gear 26, and an output shaft 28 driveably connectedto a ring gear 30. Elements are driveably connected when there is amechanical power flow path between them such that the speeds of theelements are constrained to be substantially proportional. Planetcarrier 22 supports a set of planet gears 32 such that each planet gearis in continuous meshing engagement with sun gear 26 and ring gear 30.Output shaft 28 drives the vehicle wheels directly or indirectly, suchas via a differential assembly, for example.

Traction motor 34 is driveably connected to the output shaft 28. Boththe generator 24 and the traction motor 34 are reversible electricalmachines that are capable of converting electrical power into rotationalmechanical power or converting rotational mechanical power intoelectrical power. The terms generator and motor should be regardedmerely as labels for ease of description and does not limit the functionor operation of either electrical machine. Generator 24 and tractionmotor 34 are both electrically connected to battery 36.

The rotational speed of sun gear 26, carrier 22, and ring gear 30 arelinearly related such that the speed of carrier 22 is a weighted averageof the speed of sun gear 26 and ring gear 30. Consequently, the speed ofthe engine 20 is not constrained to be proportional to the speed of theoutput shaft 28 in this arrangement. Instead, the engine speed can beselected or controlled independently of the vehicle speed by setting thegenerator speed accordingly. Power flows from the engine to the outputshaft through a combination of mechanical power transfer and electricalpower transfer. During some operating conditions, the engine 20 cangenerate more power than what is delivered to the output shaft 28 withthe difference, neglecting efficiency losses, delivered to battery 36.Under other operating conditions, the battery 36 in combination withgenerator 24 and/or traction motor 34 can supplement the power deliveredby the engine 20 such that more power is delivered to the output shaft28.

The engine 20, generator 24, and traction motor 34, all respond tocontrol signals from controller 38. These control signals determine theamount of torque generated. The controller also receives speed signalsfrom the engine 20, generator 24, and traction motor 34 and a state ofcharge signal from battery 36. The controller accepts input signalsindicating driver intention from a brake pedal 40, an accelerator pedal42, a shift lever 44, a steering wheel 46, a downshift selector 48, anupshift selector 50, and a cruise control button 51. Shift lever 44allows the driver to select Park, Reverse, Neutral, Drive, and Sportdriving modes. The upshift and downshift selectors may, for example, bepaddles mounted on opposite sides of the steering wheel. Other upshiftand downshift selector implementations, such as additional positions ofthe shift lever, are known and are suitable for use with the presentinvention.

In certain operating modes, the engine speed may vary continuously inresponse to changes in accelerator pedal position as opposed to varyingthrough discrete shift events. This terminology should not be construedto preclude use of a digital controller which manipulates a large butfinite number of control signal levels at frequent time intervals.

The top level control states are illustrated in FIG. 2. The controllerstarts in state 60 and transitions to Normal mode 62 as soon as thedriver selects the Drive (D) position using shift lever 44. Operation inNormal mode is illustrated by the flow diagram of FIG. 3. In Normalmode, the controller repeatedly performs the operations of setting theoutput torque 66, setting the engine mode 68, setting the engine power69, and setting the engine speed 70. In Normal mode, the target outputtorque is calculated at step 66 based on accelerator pedal position andvehicle speed using a table such as that illustrated in FIG. 4. Vehiclespeed can be calculated from traction motor speed or wheel speedsensors. Engine mode is set to either running or stopped at step 68using a variety of input signals including battery state of charge,output power command, accelerator pedal position, and vehicle speed. Ifthe engine mode is running, a target engine power and target enginespeed is calculated at steps 69 and 70 to minimize fuel consumptionwhile delivering the desired output torque and maintaining the batteryat a desired state of charge. If the battery state of charge is near atarget level, then the target engine power is set equal to the powerthat is to be delivered to the wheels which can be computed from thetarget wheel torque and the vehicle speed. If the battery state ofcharge is low, the target engine power is set higher to generateadditional power to charge the battery. If the battery state of chargeis high, the target engine power is set lower to conserve fuel. Thetarget engine speed is computed based on target engine power and vehiclespeed using a table such as that illustrated in FIG. 5. Finally,operating parameters of the engine, generator, and traction motor areadjusted such that the actual output torque and engine speed tend towardthe selected targets.

Referring again to FIG. 2, the controller transitions from Normal mode62 to Live-In-Drive (LID) mode 72 whenever the driver activates thedownshift selector 48. LID mode permits the driver to influence theengine speed and wheel torque by selecting a virtual gear number.Operation in LID mode is illustrated by the flow diagram of FIG. 6. Uponentering LID mode, the controller selects an initial virtual gear ratioat step 74 and then repeatedly performs the operations of setting theoutput torque at steps 76 and 66′, setting the engine power and enginespeed at steps 69′ and 78, and updating the virtual gear ratio in steps80 and 82. Each of these operations is discussed in additional detailbelow. As shown in FIG. 2, a number of conditions cause the controllerto transition back to Normal mode 62, including vehicle speed droppingbelow a low threshold value or an automatically selected downshift.Additionally, a transition can be triggered when the controller detectsa cruising condition, as indicated by activation of the cruise control51, or a tip-out condition, indicated by a reduction in acceleratorpedal position, and the condition persists for some predetermined amountof time. This latter type of condition will not result in a transition,however, if the controller detects a high driver workload at step 84,such as might be indicated by large displacements of steering wheel 46,large yaw, pitch, or roll rates, or high longitudinal or lateralaccelerations, for example.

At step 76, a modified accelerator pedal position is calculated from themeasured accelerator pedal position using a table such as illustrated inFIG. 7. This modified accelerator pedal position is used in place of theactual pedal position in step 66′ to calculate the target output torque.The curves in FIG. 7 are selected to simulate the output torquecapability of a powertrain with a discrete ratio transmission.Specifically, as the virtual gear number (1^(st) through 8^(th) in thisexample) increases, the resulting target output torque is lower for anygiven non-zero accelerator pedal position. The combined effect of steps76 and 66′ is operation of the engine and at least one traction motorsuch that combined output torque corresponds to one of a plurality ofoutput torque functions, each output torque function having a distinctoutput torque at a maximum value of accelerator pedal position for anassociated vehicle speed.

The initial virtual gear is selected at step 74. The operating pointwith respect to FIG. 7 is along line 236 prior to the transition. Thecontroller selects virtual gear number corresponding to the next highercurve from among curves 240-254. In other words, the controller selectsa virtual gear number based on the current actual pedal position suchthat, in the selected virtual gear the modified pedal position is higherthan the actual pedal position, but the modified pedal position would beless than the actual pedal position in the next higher virtual gear. Forexample, if the operating point before the transition is point 258,4^(th) gear would be selected such that the operating point becomespoint 260. This has the effect of ensuring that wheel torque increasesupon transitioning from Normal mode to LID mode at constant acceleratorposition.

As also shown in FIG. 6, in LID mode 72, the target engine power iscalculated at step 69′ and target engine speed is calculated at step 78.At step 78, the controller first calculates a clipping limit for theengine power based on the vehicle speed and the currently selectedvirtual gear number using a table such as illustrated in FIG. 8. Forexample, if the current virtual gear number is 4, the clipping limitwould be set according to curve 268. If the clipping limit is higherthan the target engine power, then the clipping limit is used in placeof the target engine power to calculate the target engine speed. Whenthe clipping limit is used, engine speed is set higher in lower virtualgear numbers than it would be set in higher virtual gear numbers. Also,when the clipping limit is used, the target engine speed does not varyas accelerator pedal position varies. When the clipping limit is lessthan the target engine power, which would be most likely when a highvirtual gear number is selected, then the target engine speed is thesame in LID mode as it would be in Normal mode. The clipping limit doesnot impact the commanded engine power which is adjusted to deliver thetarget wheel torque.

As also shown in FIG. 6, in step 80, the controller checks foractivations of either the upshift selector or the downshift selector andadjusts the virtual gear number accordingly. In step 82, the controllerdetermines if there is a need to automatically adjust the virtual gearnumber. In particular, an upshift can be triggered by an increase invehicle speed. Similarly, a downshift can be indicated when vehiclespeed decreases. However, as mentioned previously, the controllertransitions back to Normal mode 62 when an automatic downshift isindicated. In this embodiment, the automatic shift criteria arecalibrated such that automatic changes in virtual gear number aregenerally less common than shifts in a traditional discrete ratioautomatic transmission.

Referring once again to FIG. 2, the controller transitions from Normalmode 62 to Sport mode 94 whenever the driver moves the shift lever 44 tothe Sport (S) position. Operation in Sport mode is illustrated by theflow diagram of FIG. 9. The controller repeatedly performs theoperations of setting the output torque 96 and 66″, setting the enginespeed 99, and setting the engine mode 98. To provide a more sportyreaction to accelerator pedal movements, the target output torque iscomputed based on a modified accelerator pedal position as illustratedby the upper heavy line 238 in FIG. 7. The mapping between actualaccelerator pedal position and modified accelerator pedal position isselected such that the value is equal at the minimum 237 and maximum 239values, but the modified value is higher for all intermediate levels.

As also shown in FIG. 9, target engine speed is set in step 99 using asimilar algorithm to that used in Normal mode. However, the targetengine speed is scaled up by a designated amount, such as 10-20% forexample, relative to the value that would be used in Normal mode. Unlikethe algorithm for setting engine mode used in Normal mode, the algorithmused in Sport mode as indicated at step 98 only stops the engine whenthe vehicle is stationary and the brake pedal is depressed. The modifiedengine mode setting algorithm is illustrated in FIG. 10. If the engineis currently stopped 100, then the engine is restarted at step 102 ifthe vehicle is moving 104 or the brake pedal is released 106. Similarly,if the engine is currently running, then the engine is stopped at step108 only if the vehicle is stationary 110 and the brake pedal is pressed112.

If the driver activates either the upshift or downshift selector whilein Sport mode 94, the controller transitions to Select ShiftTransmission (SST) mode 114, as shown in FIG. 2. In SST mode, the targetengine torque and target engine speed are set based on the virtual gearnumber, as described with respect to LID mode. However, the controllerwill remain in SST mode until the driver indicates a desire to leavethis mode by either holding a shift selector 48 or 50 for severalseconds of by moving shift lever 44 back to the Drive (D) position.Operation in SST mode is illustrated by the flow diagram of FIG. 11. Theinitial virtual gear number is set at step 74′ using an analogous methodto that used when entering LID as described above except that theinitial operating point is along curve 238 in FIG. 7 instead of curve236. Therefore, the controller selects the highest virtual gear numberfor which the modified pedal position is higher than it would be inSport mode. This ensures an increase in wheel torque upon transitioninginto SST mode. In SST mode, the virtual gear number is adjusted at step80′ in response to activation of downshift selector 48 and upshiftselector 50 in the same manner as in LID mode. In addition, thecontroller can automatically adjust the virtual gear number, either upor down, in response to changes in vehicle speed or accelerator pedalposition. This automatic feature sets the virtual gear number to 1stgear as the vehicle comes to a stop. However, the driver can overridethis selection by manipulating the shift selectors while the vehicle isstationary in step 118. In SST mode, the engine mode depends on thevirtual gear number, vehicle speed, and accelerator pedal position. Instep 120, the controller calculates an engine shutdown limit, which isan accelerator pedal position below which electric drive is enabled. Theshutdown limit is a function of output power demand, virtual gearnumber, and vehicle speed. The shutdown limits for several gear ratiosat a particular vehicle speed and output power demand are illustrated byblack circles in FIG. 7. When one of the higher virtual gear numbers,i.e. 5th-8th, is active and the accelerator pedal position is less thanthe shutdown limit, the normal engine mode algorithm 68′ of Normal modeis used. If a lower virtual gear number, i.e. 1st-4th, is active, or ifthe accelerator position is above the engine shutdown limit, then themore restrictive algorithm 98′ of Sport and LID modes is used.

As illustrated by the representative embodiments described above,various embodiments according to the present disclosure can provide oneor more advantages, such as emulating a manual or select shift mode ofan automatic step-ratio transmission in a hybrid vehicle having acontinuously variable transmission or similar gearbox. In addition,various strategies of the present disclosure provide drivers of hybridvehicles more interactive controls to manually command powertrain speedand acceleration to provide enhanced luxury features and a sporty feel.

While exemplary embodiments are described above, it is not intended thatthese embodiments describe all possible forms of the invention. Rather,the words used in the specification are words of description rather thanlimitation, and it is understood that various changes may be madewithout departing from the spirit and scope of the invention.Additionally, the features of various implementing embodiments may becombined to form further embodiments of the invention. While variousembodiments may have been described as providing advantages or beingpreferred over other embodiments with respect to one or more desiredcharacteristics, as one skilled in the art is aware, one or morecharacteristics may be compromised to achieve desired system attributes,which depend on the specific application and implementation. Theseattributes include, but are not limited to: cost, strength, durability,life cycle cost, marketability, appearance, packaging, size,serviceability, weight, manufacturability, ease of assembly, etc. Theembodiments described herein that are described as less desirable thanother embodiments or prior art implementations with respect to one ormore characteristics are not outside the scope of the disclosure and maybe desirable for particular applications.

What is claimed is:
 1. A method of controlling a vehicle comprising:receiving signals from a speed sensor, a pedal, and a shift selector;controlling an engine and traction motor in an automatic mode to producean output torque based on a pedal position and a vehicle speed; andincreasing the output torque to transition to a select shift modewherein the output torque is based on a driver modifiable virtual gearnumber, the pedal position, and the vehicle speed.
 2. The method ofclaim 1 wherein, in the select shift mode, the output torque decreasesas virtual gear number increases.
 3. The method of claim 2 furthercomprising, when transitioning to select shift mode at constant vehiclespeed and pedal position, selecting a highest initial virtual gearnumber such that the output torque increases.
 4. The method of claim 1wherein the transition is initiated in response to operation of theshift selector.
 5. The method of claim 1 further comprising decreasingthe virtual gear number and increasing the output torque in response todriver operation of the shift selector while operating in the selectshift mode.
 6. The method of claim 1 further comprising increasing thevirtual gear number and decreasing the output torque in response todriver operation of the shift selector while operating in the selectshift mode.
 7. A controller for a hybrid electric vehicle, thecontroller comprising: communication channels configured to receivesignals indicating a vehicle speed, a position of a driver operatedaccelerator pedal, and operation of a shift selector and configured tosend signals to control an engine and at least one traction motor; andcontrol logic configured to control the engine and traction motor in anautomatic mode to produce a transmission output torque based on thepedal position and the vehicle speed; and increase the transmissionoutput torque to transition to a select shift mode wherein thetransmission output torque is based on a driver modifiable virtual gearnumber, the pedal position, and the vehicle speed.
 8. The controller ofclaim 7 wherein the control logic is configured to adjust thetransmission output torque, when operating in the select shift mode in agiven virtual gear number, such that the transmission output torque isless than it would be at the same pedal position and vehicle speed inthe next higher virtual gear number.
 9. The controller of claim 8wherein the control logic is configured to select a highest availableinitial virtual gear number such that the transmission output torqueincreases when transitioning to select shift mode at constant pedalposition and vehicle speed.
 10. The controller of claim 7 furthercomprising control logic configured to respond to operation of the shiftselector, while operating in the select shift mode, by increasing thetransmission output torque.
 11. The controller of claim 7 furthercomprising control logic configured to respond to operation of the shiftselector, while operating in the select shift mode, by decreasing thetransmission output torque.